Method for reducing sheeting and agglomerates during olefin polymerisation

ABSTRACT

The present invention relates to a method for reducing/suppressing sheeting or agglomerates during polymerisation of olefins, especially during the fluidised bed gas phase polymerisation of olefins. In particular, the present invention relates to a method for reducing/suppressing sheeting or agglomerates during the product grade transition and/or catalyst transitions occurring during polymerisation of olefins.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/014,901filed Dec. 20, 2004, which is a continuation of application Ser. No.10/220,040 filed Nov. 19, 2002, which is a 371 of PCT/GB01/00920 filedMar. 2, 2001 and which claims priority to European Application No.00430010.9 filed Mar. 6, 2000, the entire contents of each of which arehereby incorporated by reference.

The present invention relates to a method for reducing/suppressingsheeting or agglomerates during polymerisation of olefins, especiallyduring the fluidised bed gas phase polymerisation of olefins. Inparticular, the present invention relates to a method forreducing/suppressing sheeting or agglomerates during the product gradetransition and/or catalyst transitions occurring during polymerisationof olefins.

BACKGROUND OF THE INVENTION

Processes for the co-polymerisation of olefins in the gas phase are wellknown in the art. Such processes can be conducted for example byintroducing the gaseous monomer and comonomer into a stirred and/or gasfluidised bed comprising polyolefin and a catalyst for thepolymerisation.

In the gas fluidised bed polymerisation of olefins, the polymerisationis conducted in a fluidised bed reactor wherein a bed of polymerparticles is maintained in a fluidised state by means of an ascendinggas stream comprising the gaseous reaction monomer. The start-up of sucha polymerisation generally employs a bed of polymer particles similar tothe polymer which it is desired to manufacture. During the course ofpolymerisation, fresh polymer is generated by the catalyticpolymerisation of the monomer, and polymer product is withdrawn tomaintain the bed at more or less constant volume. An industriallyfavoured process employs a fluidisation grid to distribute thefluidising gas to the bed, and to act as a support for the bed when thesupply of gas is cut off. The polymer produced is generally withdrawnfrom the reactor via a discharge conduit arranged in the lower portionof the reactor, near the fluidisation grid. The fluidised bed consistsin a bed of growing polymer particles. This bed is maintained in afluidised condition by the continuous upward flow from the base of thereactor of a fluidising gas.

The polymerisation of olefins is an exothermic reaction and it istherefore necessary to provide means to cool the bed to remove the heatof polymerisation. In the absence of such cooling the bed would increasein temperature and, for example, the catalyst becomes inactive or thebed commences to fuse. In the fluidised bed polymerisation of olefins,the preferred method for removing the heat of polymerisation is bysupplying to the polymerisation reactor a gas, the fluidising gas, whichis at a temperature lower than the desired polymerisation temperature,passing the gas through the fluidised bed to conduct away the heat ofpolymerisation, removing the gas from the reactor and cooling it bypassage through an external heat exchanger, and recycling it to the bed.The temperature of the recycle gas can be adjusted in the heat exchangerto maintain the fluidised bed at the desired polymerisation temperature.In this method of polymerising alpha olefins, the recycle gas generallycomprises the monomer and comonomer olefins, optionally together with,for example, an inert diluent gas such as nitrogen or a gaseous chaintransfer agent such as hydrogen. Thus, the recycle gas serves to supplythe monomer to the bed, to fluidise the bed, and to maintain the bed atthe desired temperature. Monomers consumed by the polymerisationreaction are normally replaced by adding make up gas or liquid to thepolymerisation zone or reaction loop.

A gas fluidised bed polymerisation reactor is typically controlled toachieve a desired melt index and density for the polymer at an optimumproduction. Conditions within the polymerisation reactor have to becarefully controlled to reduce the risk of agglomerate and/or sheetformation which may ultimately lead to bed instabilities and a need toterminate the reaction and shut down the reactor. This is the reason whycommercial scale reactors are designed to operate well within provenstable operating zones and why the reactors are used in a carefullycircumscribed fashion.

Even within the constraints of conventional, safe operation, control iscomplex adding further difficulty and uncertainty if one wishes to findnew and improved operating conditions.

There is no generally accepted view as to what causes agglomerates orsheeting. Agglomerates or sheets can, for example, form when thepolymerisation temperature is too close to the polymer sinteringtemperature or when the polymer particles become excessively sticky.Highly active fine particles can, for example, concentrate in the upperelevations of the polymerisation zone, towards the top of the fluidisedbed and in the powder disengagement zone above the bed thus leading tolocal hot spots and potential agglomeration and/or sheeting.

SUMMARY OF THE INVENTION

According to the present invention a thorough understanding of sheetingand agglomeration mechanisms has allowed us to develop product specificoperating windows where sheeting or agglomeration do not occur. This isillustrated with comparative examples, that the newly developedoperating windows are unusual and that the “man skilled in the art”would previously have avoided such operation for fear of encounteringthe very operating problems that the technique overcomes.

An embodiment of the present invention finds its source in the study ofthe properties of reacting polymer particles. It has been found thatsheeting or agglomeration do not occur when instantaneous particleproperties (mechanical, physical, dielectric . . . ) are maintained in abounded window.

Industrial operating usually requires the production of differentgrades. Product transition usually corresponds to a variation inparticle properties. It is an embodiment of the present invention topropose a procedure to limit the change of critical particle propertiesduring grade transitions. This is performed by continuously changingoperating conditions such that particle properties remain in a boundedwindow during grade transition.

Agglomerates or sheeting are responsible for costly production losses,unreliable operation, strong limitations on plant performance andconsiderable damage to the global polyolefin businesses.

The present invention allows us to increase plant capacity by up to 50%for certain grades when the limitation is sheeting or agglomerates.

The shape of agglomerates or sheeting varies widely in size and aspectbut they are usually similar in most respects. One of the most probablecause of agglomeration or sheeting (when operating far from powdersintering temperature) is the accumulation of powder at the reactorwalls. We believe that the layer formed at the wall can be as thin as afew micrometers and up to several centimeters. The corresponding sheetor agglomerates have comparable thickness. The length of agglomeratescan vary between a few centimeters and several meters.

A visual inspection at the outlet of the reactor can be used in order tomonitor the presence of sheets or agglomerates. Temperature probes canalso monitor the formation of the sheets or agglomerates. The probes canbe anywhere between the insulation of the reactor (when used) to thecentre of the reactor. The analysis of temperature probes is believed tobe an excellent indication of the formation of sheets or agglomerates. Asurprising lowering of the temperature at the wall indicates thatparticles adhere, causing a probable insulating effect from the bulktemperature. Deviations of a few degrees up to more than 20° C.(sometimes 35° C.) have been commonly observed. When skin temperaturesstart to rise, it indicates the presence of a reacting layer of powderat the wall. The corresponding zone being of limited heat transfer, suchcases often lead to an agglomerate storm. Another very advantageousmonitoring tool consists in optical fibres located on the surface of thereactor, examples thereof can be found in French patent applicationn^(o)0007196 filed on 6 Jun. 2000 by BP Chemicals SNC.

It is also believed that the layer of powder at the wall (fused or not)may be able to fall into the reactor. This is observed by a cleardisruption of fluidisation patterns (pressure probes).

When sheeting or agglomeration occurs, industrial experience (and thetheory) has taught us to reduce operating temperature untilagglomeration stops. This procedure is basic and is usually used byoperators. However, it does not solve the root of the problem andagglomerates can reappear later, especially during grade transition. Thelow temperature operation is also detrimental regarding heat exchangelimitations.

More than 20 years of publications indicate that electrostaticity in thebed is the contributing factor to agglomeration at the wall. However, ananalysis of the prior art methods disclosed in the literature tend toprove that a plant control based on electrostatic measurement is notindustrially satisfactory since the electrostatic measurement tool perse is influenced by too many factors which are totally notrepresentative of fouling problems.

In this respect, the present invention indicates that the problem ofagglomeration or sheeting can be solved regardless of static electricityconsiderations.

The production losses, down time, reactor cleaning and other problemsrelated to sheeting or agglomeration are contributory to a highproportion of unplanned reactor downtime. Therefore, there is anon-going need to provide additional methods of agglomeration/sheetingcontrol.

Accordingly, the present invention provides a process forreducing/suppressing sheeting or agglomerates during polymerisation ofolefins, especially during the fluidised bed gas phase polymerisation ofolefins. In particular, the present invention relates to a method forreducing/suppressing sheeting or agglomerates during start-up,transitioning and steady state olefin polymerisation.

This paragraph summarises the approach used to define the optimumoperating window for polymer particle properties according to thepresent invention.

The Applicants have found that numerous grade transitions and start-upprocedures in industrial operation are characterised by drastic changesin instantaneous particle properties which lead to agglomerates and/orsheeting at the reactor wall.

A stochastic model of the fluidised bed based on a refined Monte-Carloapproach has been built in order to help understand potentialagglomeration mechanisms.

The behavior of a representative set of 10 million particles issimulated in order to elevate the amount of overheating particles, i.e.those particles for which the surface temperature is higher thansintering temperature, i.e. the temperature which is slightly inferiorbelow the melting temperature and which is representative of thetemperature at which the polymer powder starts to agglomerate. For thepurpose of the present description and appended claims, the sinteringtemperature of the polymer powder under reactor operating conditions isthe temperature at which a bed of said polymer powder in contact with agas having the same composition as the reactor recycle gas used inproducing the polymer powder will sinter and form agglomerates whenfluidization velocity is at maximum taking into account the fineparticle entrainment limitation. The sintering temperature is decreasedby decreasing the resin density, by increasing the melt index and byincreasing the amount of dissolved monomers.

The particle temperature is estimated by solving heat transfer equationsat the level of the particle. The fundamental mechanisms involved inthat process can be divided in 2 categories: mechanisms responsible forheat generation (polymerisation reaction depending on well quantifiedkinetics) and equations governing heat transfer.

Heat generation is well quantified based on well known reaction kineticsand the stochastic approach allows us to describe the complexity of thefluid bed reactor using statistical dispersion of key parameters (suchas partial pressure of reactants, initial concentration of active sites,level of impurities, . . . ) around their quasi-steady state averagevalues. This process allowed us to generate a representative set ofreacting particles in the reactor (10⁷).

Heat transfer quantification is more complex to quantify due tocompetition between the different mechanisms involved: for eachparticle, heat transfer is quantified by considering local gas velocityat the level of the particle (governed mainly by particle size andposition in the reactor), vaporisation of liquid at the surface of theparticle (in liquid injection mode, e.g. condensation mode) and gascomposition, pressure and temperature. As for heat generation, astochastic approach is used to simulate the fluidised bed behaviour.

FIG. 2 illustrates the typical results obtained for gas phasepolymerisation process wherein the mass percentage of overheatingparticles is given for increasing polymerisation temperature.

At very high polymerisation temperature, operation is too close to thesintering temperature of the powder and particles massively agglomerate.Operation is highly unstable and the risk of agglomerating the entirebed is high. Operators are constantly aware of this danger and keepoperation far away from the powder sintering limit, i.e. in the “commonoperating window”.

However, the typical Overheating/Temperature curve also indicates thatagglomerates can be formed at lower temperature and points out theexistence of a local minimum where temperature is still high but therisk of agglomeration or sheeting is very low.

The corresponding operating window is the optimumagglomerate/sheeting-free operating window (as indicated on the righthand side of FIG. 2), which is also called the high temperature optimumoperating window.

In fact, operators being aware of the risk of agglomeration at hightemperatures prefer to operate with a significantly safety margin atmuch lower temperatures than the sintering temperature. It is clear thatthere is a resistance in the art to increasing operating temperaturethrough a fear of encountering powder sintering limits. However, thepresent invention demonstrates that it is possible by acting againstthis natural tendency, i.e. by increasing the operating temperature, tocontrol advantageously the polymerisation while reducing and/oreliminating the agglomeration/sheeting risks.

It is therefore an object of the present invention to provide a processfor reducing/suppressing sheeting or agglomerates during polymerisationof olefins, characterised in that the operating temperature iscontrolled in order to maintain the polymer particle in its hightemperature optimum operating window throughout the polymerisation.

Indeed, once the man skilled in the art is aware of the existence ofsaid optimum operating window, he is able to control his plant, and inparticular the operating temperature, in such a way that the polymerparticles remain in said optimum operating window.

This process is preferably applied during the fluidised bed gas phasepolymerisation of olefins, especially during start-up and transition,more preferably during product grade transition.

While not wishing to be bound by the theory, the explanation for theexistence of an increasing risk of sheeting/agglomeration at lowtemperature is related to instantaneous reacting particle properties.Indeed, temperature highly affects instantaneous particle properties(mechanical, physical and dielectric). When particle temperature isdecreased (this can be done by decreasing polymerisation temperature),particles become more brittle, and surface properties are modified.

At low temperature, the generation of fines and micro-fines drasticallyincreases. Although the fines fraction represents a low percentage inmass, it represents a considerable number of particles which aresusceptible to adhere to the reactor wall due to their small size.

Conversely, when operating temperature is controlled in order to remainin the high temperature window throughout the polymerisation, it hasbeen unexpectedly found that (micro-)fines generation could be loweredat a level where the presence of said (micro-)fines did not entrain anyirreversible agglomeration phenomenon.

The stochastic model for the fluidised bed also pointed out the simplefact that agglomerates or sheeting are formed when heat exchange islimited. When this is the case, the fraction of overheating particles ishighly dependent on operating parameters such as condensation rate,fluidisation velocity, polymerisation rate (heat generated) andprepolymer or catalyst fines. On the contrary, when operating in theoptimum window for particle properties, heat exchange is not limitingand the operating conditions previously mentioned do not affect thefraction of overheating particles, to the same extent. In that case,plant performance can be increased by pushing catalyst productivity andproduction rate. (FIG. 3)

The last observation to be mentioned concerns the most commonly usedoperating window which is the so called “low temperature window” (lefthand of FIG. 2). It corresponds to the case where operating temperatureis sufficiently low so powder does accumulate at the wall but particleoverheating remains controllable. This operating window can beconsidered as metastable. Although it is the commonly used operatingwindow, we have found that it is non-optimised in many respects: heatexchange capacity is limited, agglomerates or sheeting can form whenoperating conditions are changed or production rate is increased, andmost likely during grade transitions when particle properties aresignificantly changed.

Another object of the present invention, which is illustrated in detailin the following examples, relates to a process for reducing/suppressingsheeting or agglomerates during transition between two different polymerproducts made during the polymerisation of olefins, characterised inthat the operating temperature is controlled in order to maintain thepolymer particle in its high temperature optimum operating windowthroughout the transition.

In order to further analyse the corresponding phenomena, a criteria hasbeen used to follow the changes in particle properties in real time,i.e. the instantaneous particle properties.

The particle properties regarded as important are the following:toughness, brittleness, crystallinity, conductivity, softeningtemperature, and sintering temperature.

Amongst the different possibilities, a combined criteria has beenselected for the following reasons:

It varies with polymer crystallinity

It is a marker of polymer dielectric properties

It is derived from a mechanical property (Tensile Strength).

The general form of the criteria is the following:

Structure/Property Models are used to predict resin properties in-realtime in order to build an on-line criteria for monitoringagglomerate/sheeting-free operating windows.

Resin properties are predicted from resin molecular structure which isrelatively simple in the case of simple polymers such as polyethylene orpolypropylene.

In the following description, the examples of Linear Low DensityPolyethylene (LLDPE) and High Density Polyethylene (HDPE) will becovered. However, it is clear that the generality of the definedcriteria is applicable to a large range of applications.

Molecular Structure for LLDPE/HDPE:

The simple molecular structure in that case can be described by theaverage polymer chain length, the dispersion of the chain lengths(polydispersity), the type of short chain branching (type of comonomer),the amount of short chain branching, the short chain branchingdistribution, and the size and amount of long chain branching.

In practice, all of this information is not necessary to predict resinproperties to sufficient accuracy when the range of products beingconsidered is limited (e.g. to certain catalyst types or even specificcomonomers). A limited set of relevant parameters have in thesecircumstances been found to be highly sufficient. Indeed, from a Processmonitoring point of view, the simplest description of resin molecularstructure is highly desirable: for a given catalyst and comonomer typethe first order parameters to be considered are the average polymerchain length and the amount of comonomer. Consequently, the simplestapproach is to use Melt-Index (average molecular weight) and Density(amount of comonomer) to describe the changes in resin molecularstructure. The Criteria “Crit” will depend on the specifics of thedifferent comonomer types and catalysts.

The main difficulty in the Structure/Properties approach is theprediction of particle properties in reacting conditions.

This problem has been solved from a process monitoring point of view byquantifying the effects of the most sensitive parameters only. These arethe parameters having a great influence on particle properties in theusual range of variation in industrial operation. For instance, thecriteria “Crit” will be modified in order to differentiate plantoperating at high polymerisation rates. Indeed, a high polymerisationrate will affect particle properties via the particle temperature whichis an important parameter for particle properties. However, the effectof this parameter being of second order, it is not mandatory toincorporate it in a more detailed model.

Example of Structure/Property Model: Particle Tensile Strength

In this example, a so called “Particle Tensile Strength” property ispredicted from resin molecular structure (Melt-Index and Density in thatcase). It is an extrapolation of Resin Tensile Strength atpolymerisation temperature.

The model has been built from measurements of Tensile Strength performedon injection moulded samples (ASTM n^(o) D638-89). Over 150 samples havebeen tested covering a wide range of densities and Melt-Indexes. Thecomparison between predictions and measurements is given in FIG. 1 forRIGIDEX™ product types.

Such models being available, particle properties in the reactor can bemonitored on-line via the prediction of Melt-Index and Density in realtime. We should take the opportunity to mention here that the criticalparticle properties involved in the agglomeration mechanisms are the socalled “instantaneous properties” which correspond to the properties ofthe resin formed instantaneously in the reacting conditions at a giventime. The “instantaneous properties” are different from the pelletproperties which correspond to a mixture of different resins formedcontinuously in the fluidised bed (averaging effect). The “instantaneousproperties” require the use of accurate process models able to predictpowder properties from operating parameters.

By taking into consideration the above, another embodiment of thepresent invention is to provide an effective process forreducing/suppressing sheeting or agglomerates during polymerisation ofolefins, process characterised in that the above criteria “Crit” ismaintained in a bounded window which corresponds to the high temperatureoptimised operating window.

Thus, the optimum operating window can be reached by controllinginstantaneous particle properties, preferably mechanical properties,e.g. tensile strength as described hereabove.

It is therefore an object of the present invention to provide a processfor reducing/suppressing sheeting or agglomerates during polymerisationof olefins, characterised in that the instantaneous properties of thegrowing polymer particles formed throughout the polymerisation aremaintained such that there is no irreversible formation of agglomeratesthrough generation of (micro-)fines.

It is a further object of the present invention to provide a process forreducing/suppressing sheeting or agglomerates during transition betweentwo different polymer products made during polymerisation of olefins,characterised in that the instantaneous properties of the growingpolymer particles formed throughout the transition are maintained suchthat there is no irreversible formation of agglomerates throughgeneration of (micro-)fines.

Indeed, once the man in the art is aware of the existence of the hightemperature optimum operating window according to the present invention,i.e. where there is no irreversible formation of agglomerates throughgeneration of (micro-)fines, he will take automatically all necessarysteps in order to maintain the instantaneous properties of the growingpolymer particles in its safe optimised window.

According to a preferred embodiment of the present invention, and asexplained hereabove, the instantaneous properties of the growing polymerparticles are predicted by using a structure/property model.

According to another preferred embodiment of the present invention, theinstantaneous properties are mechanical properties of the growingpolymer.

According to a further preferred embodiment of the present invention, itis the instantaneous tensile strength of the growing polymer particleswhich is maintained in its safe optimised window.

Polymerisation rate and fluidisation velocity may slightly influencethese criteria's.

For example, in the case of the tensile strength property criteria, fora fluidised bed polymerisation, when condensation is used, or kineticsare smoother and fluidisation velocity is higher, the operating windowis wider and therefore the optimum operating window corresponds tohigher values of the criteria.

At temperature close to the sintering temperature, the criteriadecreases rapidly to take into account the softening of the particle andthe loss of mechanical toughness (and brittleness).

One of the main advantages according to the present invention is thatthe man skilled in the art has now at his disposal a practical toolwhich allows him to determine the optimum operating window, and inparticular the optimum temperature in order to avoid sheeting oragglomerates during the polymerisation of olefins, preferably during thefluidised bed gas phase polymerisation of olefins, in particular duringpolymer product transition.

In particular, once the man skilled in the art is able to produce onepolymer grade in said optimum operating window, i.e. once he is in theposition of fulfilling the above instantaneous particle propertycriteria's, he is also automatically able to proceed efficiently withpolymer grade transitioning by keeping the said criteria at more or lessthe same value through the control of the operating temperature, asdisclosed in the examples.

It is another embodiment of the present invention to provide for analternative method for determining the high temperature optimumoperating window of a polymer A having a density A (d_(A)), a melt indexA (MI_(A)) and a sintering temperature T_(SA) which is produced at anoperating temperature A (T_(A)) characterised in the following steps:

-   -   1. monitor sheet formation    -   2. if sheet are (being) formed, increase the temperature to a        value T_(X) which is equal to or higher than        [0.5*(T_(A)+T_(SA))] and lower than the sintering temperature of        the formed polymer minus two degrees centigrade    -   3. if sheets are not (being) formed, the actual polymerisation        temperature becomes part of the high temperature optimum        operating window of the polymer A under the existing        polymerisation conditions.        Optionally, just before or just after step 2, if the sheet        formation process can not be effectively controlled, proceed        with a complete polymerisation stop process and restart the        polymerisation at a temperature which is at least equal to        T_(A), preferably at least equal to T_(X).

It is a further embodiment of the present invention to provide for analternative method for determining the optimum operating window of atransition polymer AB during the transition between a polymer A (d_(A),MI_(A), sintering temperature T_(SA), produced under temperature T_(A))to a polymer B (d_(B), MI_(B), sintering temperature T_(SB)) wherein thesaid transition polymer AB (d_(AB), MI_(AB)) is being formedcharacterised in the following steps:

-   -   1. monitor sheet formation    -   2. if d_(B)>d_(A) and MI_(B)≦MI_(A), increase the polymerisation        temperature to a value T_(X1) which is equal to or higher than        [0.5*(T_(A)+T_(SB))] and lower than the sintering temperature of        the formed polymer minus two degrees centigrade    -   3. if sheet are (being) formed, continue to increase the        temperature to a value T_(X2) higher than [0.5*(T_(X1)+T_(SB))]        and lower than the sintering temperature of the formed polymer        minus two degrees centigrade    -   4. if sheets are not (being) formed, the actual polymerisation        becomes part of the high temperature optimum operating window of        the transition polymer AB under the existing polymerisation        conditions.        Optionally, step 3 can be repeated by replacing T_(X1) by T_(X2)        in the equation.

Optionally, just before or just after step 2, if the sheet formationprocess can not be effectively controlled, proceed with a completepolymerisation stop process and restart the polymerisation at atemperature which is at least equal to T_(X1).

Once the above transitioning process has been completed and the d_(B)MI_(B) values of polymer B reached, i.e. when polymer B is successfullyproduced without sheet, then the actual polymerisation temperaturebecomes part of the high temperature optimum operating window of thepolymer B under the existing polymerisation conditions.

It is a further embodiment of the present invention to provide for analternative method for determining the optimum operating window of atransition polymer AB during the transition between a polymer A (d_(A),MI_(A), sintering temperature T_(SA), which is produced at T_(A)) to apolymer B (d_(B), MI_(B), sintering temperature T_(SB)) wherein the saidtransition polymer AB (d_(AB), MI_(AB)) is being formed characterised inthe following steps:

-   -   1. monitor sheet formation    -   2. if d_(B)<d_(A) and MI_(B)≧MI_(A), decrease the polymerisation        temperature to a value T_(Y1) equal to or higher than        [T_(SB)−1.2*(T_(SA)−T_(A))] and lower than the sintering        temperature of the formed polymer minus two degrees centigrade    -   3. if sheet are (being) formed, increase the polymerisation        temperature to a value T_(Y2) equal to or higher than        [0.5*(T_(Y1)+T_(SB))] and lower than the sintering temperature        of the formed polymer minus two degrees centigrade    -   4. if sheets are not (being) formed, the actual polymerisation        temperature becomes part of the high temperature optimum        operating window of the transition polymer AB under the existing        polymerisation conditions.

The process according to the present invention is particularly suitablefor the manufacture of polymers in a continuous gas fluidised bedprocess. Illustrative of the polymers which can be produced inaccordance with the invention are the following:

SBR (polymer of butadiene copolymerised with styrene),ABS (polymer of acrylonitrile, butadiene and styrene),nitrile (polymer of butadiene copolymerised with acrylonitrile),butyl (polymer of isobutylene copolymerised with isoprene),EPR (polymer of ethylene with propylene),EPDM (polymer of etylene copolymerised with propylene and a diene suchas hexadiene, dicyclopentadiene or ethylidene norborene),copolymer of ethylene and vinyltrimethoxy silane, copolymer of ethyleneand one or more of acrylonitrile, maleic acid esters, vinyl acetate,acrylic and methacrylic acid esters and the like.

In an advantageous embodiment of this invention, the polymer is apolyolefin preferably copolymers of ethylene and/or propylene and/orbutene. Preferred alpha-olefins used in combination with ethylene and/orpropylene and/or butene in the process of the present invention arethose having from 4 to 8 carbon atoms. However, small quantities ofalpha olefins having more than 8 carbon atoms, for example 9 to 40carbon atoms (e.g. a conjugated diene), can be employed if desired. Thusit is possible to produce copolymers of ethylene and/or propylene and/orbutene with one or more C₄-C₈ alpha-olefins. The preferred alpha-olefinsare but-l-ene, pent-l-ene, hex-l-ene, 4-methylpent-l-ene, oct-l-ene andbutadiene. Examples of higher olefins that can be copolymerised with theprimary ethylene and/or propylene monomer, or as partial replacement forthe C₄-C₈ monomer are dec-1-ene and ethylidene norbornene.

According to a preferred embodiment, the process of the presentinvention preferably applies to the manufacture of polyolefins in thegas phase by the copolymerisation of ethylene with but-l-ene and/orhex-l-ene and/or 4 MP-1.

The process according to the present invention may be used to prepare awide variety of polymer products for example linear low densitypolyethylene (LLDPE) based on copolymers of ethylene with but-1-ene,4-methylpent-1-ene or hex-1-ene and high density polyethylene (HDPE)which can be for example copolymers of ethylene with a small portion ofhigher alpha olefin, for example, but-1-ene, pent-1-ene, hex-1-ene or4-methylpent-1-ene.

When liquid condenses out of the recycle gaseous stream, it can be acondensable monomer, e.g. but-1-ene, hex-1-ene, 4-methylpent-1-ene oroctene used as a comonomer, and/or an optional inert condensable liquid,e.g. inert hydrocarbon(s), such as C₄-C₈ alkane(s) or cycloalkane(s),particularly butane, pentane or hexane.

The process is particularly suitable for polymerising olefins at anabsolute pressure of between 0.5 and 6 MPa and at a temperature ofbetween 30° C. and 130° C. For example for LLDPE production thetemperature is suitably in the range 75-110° C. and for HDPE thetemperature is typically 80-125° C. depending on the activity of thecatalyst used and the polymer properties desired.

The polymerisation is preferably carried out continuously in a verticalfluidised bed reactor according to techniques known in themselves and inequipment such as that described in European patent application EP-0 855411, French Patent No. 2,207,145 or French Patent No. 2,335,526. Theprocess of the invention is particularly well suited to industrial-scalereactors of very large size.

The polymerisation reaction may be carried out in the presence of acatalyst system of the Ziegler-Natta type, consisting of a solidcatalyst essentially comprising a compound of a transition metal and ofa cocatalyst comprising an organic compound of a metal (i.e. anorganometallic compound, for example an alkylaluminium compound).High-activity catalyst systems have already been known for a number ofyears and are capable of producing large quantities of polymer in arelatively short time, and thus make it possible to avoid a step ofremoving catalyst residues from the polymer. These high-activitycatalyst systems generally comprise a solid catalyst consistingessentially of atoms of transition metal, of magnesium and of halogen.The process is also suitable for use with Ziegler catalysts supported onsilica. The process is also especially suitable for use with metallocenecatalysts in view of the particular affinity and reactivity experiencedwith comonomers and hydrogen. The process can also be advantageouslyapplied with a late transition metal catalyst, i.e. a metal from GroupsVIIIb or Ib (Groups 8-11) of the Periodic Table. In particular themetals Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt are preferred, especiallyFe, Co and Ni. The late transition metal complex may comprise bidentateor tridentate ligands, preferably coordinated to the metal throughnitrogen atoms. As examples are those complexes disclosed in WO96/23010.Suitable iron and/or cobalt complexes catalysts can also be found inWO98/27124 or in WO99/12981.

It is also possible to use a high-activity catalyst consistingessentially of a chromium oxide activated by a heat treatment andassociated with a granular support based on a refractory oxide.

The catalyst may suitably be employed in the form of a prepolymer powderprepared beforehand during a prepolymerisation stage with the aid of acatalyst as described above. The prepolymerisation may be carried out byany suitable process, for example, polymerisation in a liquidhydrocarbon diluent or in the gas phase using a batch process, asemi-continuous process or a continuous process.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will now be described,by way of example, with reference to FIGS. 1-6D.

FIG. 1 is a graph comparing predicted tensile strength with measuredtensile strength of RIGIDEX™ type products;

FIG. 2 is a graph that illustrates a comparison between the percentageof overheating particles and polymerisation temperature in a gas phasepolymerisation process;

FIG. 3 is a graph similar to FIG. 2 showing two different heat transfercurves;

FIG. 4 is a graph similar to FIG. 2 showing the Optimum Operating Windowduring transitioning from one polymer to another;

FIGS. 5A-5D shows the results of Comparative Example 1; and

FIGS. 6A-6D shows the results of Example 2.

The following examples illustrate the present invention.

EXAMPLES

The following examples were conducted in a conventional fluidised bedreactor. The catalyst used was a Ziegler type, titanium based catalyst(supported or pre-polymerised). The products made in the examples werecopolymers of ethylene and butene, and ethylene and 4-methyl-pentene-1.Hydrogen was used as a chain transfer agent to control the melt-index ofthe polymer.

The following examples are illustrations of the monitoring ofsheeting/agglomerate-free operating window. They correspond to a boundedwindow for instantaneous reacting particle properties. The mostsensitive parameter to adjust reacting particle properties for a givenproduct is operating temperature (final pellet Melt-Index and Densitybeing set for each product type).

The following examples will illustrate the use of operating temperatureas a means to control reacting particle properties. The first example isan illustration of operating conditions moving out of the optimisedparticle properties window. It is a comparative example whichillustrates the irreversible formation of sheets/agglomerates throughgeneration of (micro-)fines at the reactor wall when particle propertiesare outside the optimum operating window.

The second example is an illustration of the optimum control of particleproperties to avoid sheeting and agglomerates. This example is a producttransition similar to the case of example 1. In this second case,temperature is adjusted to compensate for final resin property changes.This second example is an illustration of continuous operation in thesheeting/agglomerates-free operating window.

The third example is taken from WO99/02573. It is similar to the secondexample in terms of particle properties and final resin properties. Thisexample is an illustration of particle properties moving outside theoptimum window during grade transitioning. In this example, themeta-stable window has been chosen: the powder accumulation problem isnot solved but polymerisation temperature is decreased such that thelayer of powder at the wall does not melt.

Comparative Example 1

Particle properties moving outside the optimum window during gradetransitioning.

A fluidised bed reactor was transitioned from a 0.926 density, 0.6 meltindex ethylene/4-methyl-pentene-1 copolymer to a 0.935 density, 0.5 meltindex ethylene/4-methyl-pentene-1 copolymer. The prepolymer (Zieglertitanium based catalyst) was the same for both products. The bedtemperature was slightly decreased from 86° C. to 83° C. duringtransition to the higher density product.

The transition was smooth but as the 0.926 density, 0.6 Melt-Indexmaterial was replaced by the 0.935, 0.9 Melt-Index resin, walltemperature started to peak in the lower part of the reactor as aconsequence of the formation of a fused layer of powder at the wall.Later on agglomerates started to block withdrawal lines.

In this case the tensile strength criteria is used to monitorinstantaneous reacting particle properties: the first product operatingconditions correspond to particle properties in the optimum operatingwindow (no sheeting nor agglomerates). During grade transitioning, thecriteria started to increase from 5.6 to 6.5 which is outside theoptimum window. The polymer instantaneously formed in the reactor becametoo brittle and fines and micro-fines started to form. Powder thenaccumulated at wall leading to overheating as it was observed on skintemperature probes and sheeting.

This typical problem of particle properties above the upper limit of theoptimum window has been permanently solved by sufficiently increasingpolymerisation temperature (in this case 95° C. so the criteria equals5.6) as it is illustrated in the following example.

Example 2

Particle properties are maintained in the optimum operating windowduring grade transitioning.

A fluidised bed reactor was transitioned from a 0.919 density, 0.9 meltindex ethylene/butene copolymer to a 0.926 density, 0.75 melt indexethylene/butene copolymer. The prepolymer (Ziegler titanium basedcatalyst) was the same as the one used in comparative example 1. The bedtemperature was increased from 86° C. to 96° C. during transition to thehigher density product with a rate such that the tensile strengthcriteria is maintained at 5.6.

Polymerisation temperature is increased to maintain particle propertiesin the optimum window: not too close to sintering and not toobrittle/crystalline. For comparison, if polymerisation temperature hadbeen maintained at 86° C. during transition, the criteria would havereached 6.7 indicating that particle properties were far above the upperlimit of the optimum window (similar to example 1).

With such a transition procedure, particle properties remain in theoptimum window: no agglomerates/sheeting occurred and skin temperatureprobes remained at their baseline indicating that the reactor walls wereclean.

Comparative Example 3

Particle properties are moving outside the optimum window during gradetransitioning.

This example is taken from WO99/02573: the case is comparable to theprevious example which has been chosen for comparison.

A fluidised bed reactor was transitioned from a 0.917 density (insteadof 0.919 for example 2), 0.6 melt index (instead of 0.9 for example 2)ethylene/hexene copolymer to a 0.925 density (instead of 0.926 forexample 2), 0.5 melt index (instead of 0.75 for example 2)ethylene/hexene copolymer. The catalyst (Ziegler titanium-based) was thesame for both products. The bed temperature was increased from 86° C. to91° C. during the transition to the higher density product.

We have used the same tensile strength criteria to monitor instantaneousparticle property changes during the transition: the first product ismade at 86° C. which corresponds to a criteria of 5.5. This product istherefore in its optimum operating window thus explaining that neithersheeting nor agglomerates have been experienced in this case. For thesecond product, the criteria reaches 6.2 which is outside the optimumwindow for particle properties. In fact, the value of 5.6 would requireus to operate at 97° C. (comparable to the similar case reported in theprevious example). At 91° C., particle properties are too brittle andcrystalline leading to the formation of a layer of powder at the wall.Unfortunately at 91° C., the temperature is high enough so the layer ofpowder can fuse and sheets start to form. Lowering operating temperatureprevents the fusion of the layer but does not solve the problem ofinadapted particle properties.

The change of particle surface properties is probably the reason for theincrease of static level during transition: when the film starts toform, additional static is generated, and lowering operating temperatureonly stops this phenomena without solving the problem of particleproperties: metastable operating conditions are reached with all thelimitations we have described earlier: heat transfer capacity, and highsensitivity to operating parameters such as condensation, fluidisationvelocity, polymerisation rate, and production rate regardingsheeting/agglomerates problem.

This last example is an excellent illustration of the use of theparticle properties criteria to monitor the sheeting and agglomeratesfree operating window. It underlines that the finding of this window isa breakthrough which was not obvious for the “Man of the Art” as itrequires to move counter to the prejudice of operating closer to powdersintering temperature. The criteria used to determine the optimumoperating window has proved to be extremely powerful as it alsodetermines the position of the optimum window not only for steady-stateoperation but at any time during transitions and start-ups as well.

Example 4

The catalyst used was 2,6-diacetylpyridinebis(2,4,6-trimethylanil)FeCl₂activated with methylaluminoxane (MAO) and supported on silica(Crosfield grade ES70X). The preparation of this catalyst is describedin detail in WO 99/46304, the content of which is incorporated herein byreference.

The polymerization was carried out in a conventional fluidized bed gasphase polymerization reactor. The catalyst injection rate was set suchas to maintain the production rate constant at the desired level. Duringthe production of an ethylene polymer at a polymerization temperature of90° C., cold bands on the reactor wall were observed; the polymerizationtemperature was consequently increased to 96° C. and, within a shortperiod of time, disappearance of cold bands could be observed which issynonymous of having reached the optimum operating window.

1. Process for reducing/suppressing sheeting or agglomerates during polymerisation of olefins, characterised in that the operating temperature is controlled in order to maintain the polymer particle in its high temperature optimum operating window throughout the polymerisation.
 2. Process for reducing/suppressing sheeting or agglomerates during transition between two different polymer products made during the polymerisation of olefins, characterised in that the operating temperature is controlled in order to maintain the polymer particle in its high temperature optimum operating window throughout the transition.
 3. Process for reducing/suppressing sheeting or agglomerates during polymerisation of olefins, characterised in that the instantaneous properties of the growing polymer particles formed throughout the polymerisation are maintained such that there is no irreversible formation of agglomerates through generation of (micro-) fines.
 4. Process for reducing/suppressing sheeting or agglomerates during transition between two different polymer products made during polymerisation of olefins, characterised in that the instantaneous properties of the growing polymer particles formed throughout the transition are maintained such that there is no irreversible formation of agglomerates through generation of (micro-)fines.
 5. Process according to claim 3 wherein the instantaneous properties of the growing polymer particles are predicted by using a structure/property model.
 6. Process according to claim 3 wherein mechanical properties are used as instantaneous properties of the growing polymer.
 7. Process according to claim 6 wherein tensile strength is used as the instantaneous property of the growing polymer particles.
 8. Process according to claim 3 wherein the instantaneous properties of the growing polymer particles formed throughout the polymerisation are maintained such that there is no irreversible formation of agglomerates through generation of (micro-) fines through control of the operating temperature. 